An introductory section that sets up a lengthy text is a

a. Timeline.

b. Conclusion.

c. Prologue.

d. Caption.

I believe it's C. Prologue.

c. Prologue.

Explanation:

The prologue is an introduction to a long text. This can be written by the same writer of the text or by any other writer. The aim of the prologue is to invite the reader to read the whole text since it is worth doing so. To sound that convincing, the different writers may say something about the background of the text: the reasons why it has been written; the way it can be compared to other famous written pieces and / or the good qualities the writer of the text has - this is expressed by the other writer not by the author of the piece in question-.